When beneficiating minerals by flotation, it is necessary that this mineral be preliminarily comminuted to the size of solids allowing to carry out the process of flotation. The optimum size of solids of the useful ingredient capable of floating up from the volume of the flotation pulp is different for each type of mineral, and depends largely on the density of the useful ingredient in such a mineral.
For example, with regards to an ore mineral beneficiated by widely known flotation machines, the average size of solids normally ranges from 0.01 to 0.1 mm. For a diamond-containing mineral, the optimum size of particles capable of floating up from the body of the flotation pulp is not more than 0.5 mm.
Reducing a mineral to the optimum particle size is accompanied by excessive comminution of the useful ingredient disseminated in the mineral to a size which is more than the upper limit of floatability, or to a size which is close to the optimum. As is known, reduction in the size of solid particles of the useful ingredient affects the value of such a useful ingredient. Such a loss of value is especially pronounced when overcomminuting a diamond-containing mineral.
It is also to be noted that the greater part of the overall expenditures associated with beneficiation of minerals falls on comminution, and is as large as 40% of all expenditures for processing the mineral.
Therefore, it is especially important to increase the upper limit of the size of mineral particles subjected to processing in a flotation machine. The accompanying advantage is an increase in the efficiency of the equipment for comminuting minerals. For example, an increase in the upper limit of particle size results in a 30% growth in the efficiency of ball mills. In some instances a higher grain size concentrates are more amenable to subsequent processing. Large diamond crystals have a higher value than small ones.
There is known a flotation machine (cf., SU, A, 984,498) comprising a vertical cylindrical chamber for circulating a flotation pulp having a tapered bottom and accommodating at the top part thereof a trough for collecting froth concentrate, and a pipe for continuously feeding the flotation pulp positioned axially of the chamber. Disposed coaxially inside the chamber is a hollow cone member with the top of the cone facing the bottom of the chamber, this cone member having slotted holes to distribute uniformly the pulp in the volume of the chamber. The slotted holes are spaced at equal distances from one another in terms of the height of the cone, inclined to the axis of the cone at an acute angle, and directed toward the upper end of the chamber.
The bottom part of the chamber accommodates pulp aerators in the form of perforated rubber tubes, and a pipe for discharging gangue.
The top size limit of particles of the useful ingredient of the mineral capable of floating up from the body of aerated pulp with respect to a diamond-containing mineral is not more than 1 mm. In other words, the maximum size of solid particles of the useful ingredient in a froth concentrate produced in this flotation machine is not more than 1 mm.
There is also known a flotation machine capable of doubling the size of solids in a froth concentrate (cf., SU, A, 1,183,180).
This flotation machine includes a vertical cylindrical chamber to circulate a flotation pulp having a tapered bottom to which there are secured a pipe for feeding the flotation pulp containing mineral particles of fine fraction and a pipe for discharging gangue, an annular trough for collecting froth concentrate attached to the walls of the pulp circulation chamber in its top portion, a group of tapered shells secured axially inside the pulp circulation chamber and spaced at equal distances from one another in terms of the height of the chamber, the height and inclination angles of the generating lines of the tapered surfaces thereof to their axes of rotation being substantially equal, bases of larger diameter of the shells facing the top of the chamber and resting in one tapered surface outside the tapered shells, the inclination angle of the generating line of this common tapered surface to its own axis of rotation being smaller than the inclination angle of the generating lines of the tapered surfaces of the shells, at least one group of pulp aerators having tubular casings thereof secured at the walls of the pulp circulation chamber and spaced equidistantly about the circumference, and a means for feeding mineral particles of coarse fraction positioned over the pulp circulation chamber.
Provision of this means for feeding mineral solids of coarse fraction to the froth layer makes it possible to obtain a froth concentrate containing solid particles of a diamond-containing mineral up to 2 mm across, as the froth layer of the pulp is capable of reliably holding solids of the useful ingredient of the mineral of a size at least twice the size of solid particles of the useful ingredient of the mineral capable of floating up from the body of the aerated pulp.
However, this flotation machine suffers from losses of quite large solids of the useful ingredient which can fall out of the froth layer as it moves axially of the chamber toward the froth collecting trough. Because the size of these solid particles of the useful ingredient of the mineral entering the annular clearance between the tapered shells and walls of the cylindrical pulp circulation chamber exceeds the upper size limit of particles capable of floating up from the volume of the aerated pulp, such particles are irretrievably lost when entrained by the gangue. Return to the froth layer of particles of useful ingredient of the mineral sizing close to the upper size limit of solids capable of floating up from the body of the aerated pulp and entering this clearance is very unlikely, since the vectors of velocity of air bubbles and mineral solids are directed to the opposite sides to result in a smaller tendency of air bubbles to adhere to the surface of the solid particles of the useful ingredient of the mineral.
In this prior art flotation machine for beneficiating minerals irretrievable losses of the particles of useful ingredient of the mineral sizing close to the upper size limit when such particles are still capable of floating up from the body of the aerated pulp bring down the yield of the useful ingredient from the mineral. This results in reduced percentage of large-size particles in the froth concentrate affecting the froth concentrate obtained in the course of beneficiation of a diamond-containing mineral.